Here's why:
* Dragon Assault's effect: It states "When a monster is Normal Summoned or flipped face-up: Destroy 1 monster on the field."
* Special Summons: Special summons are a different method of summoning monsters than Normal Summoning or flipping face-up. They are not affected by Dragon Assault's effect.
Therefore, Dragon Assault only affects monsters summoned through Normal Summoning or flipping face-up.